Ask the Irlbachers (Archive)

Paul K.: “How is glass made?”

Martin Kreuzer, Operations Manager

The primary material of practically all optically transparent glass is silicon dioxide (SiO2) – quartz sand, like on white beaches. The finer and the purer, the better for glass production. The problem involved: Pure quartz sand has to be heated to over 2,000 °C in order to melt and be processed as quartz glass.

To lower this melting temperature significantly, so-called fluxes are added to the quartz sand. “Window glass”, the most common soda-lime glass in the world, consists of about 70% SiO2, plus about 15% sodium oxide (Na2O) and about 10% calcium oxide (CaO; unslaked lime). The remaining approx. 5% is made up of additives that are used to adjust certain properties of the glass, such as its colour. With around 1,100 °C, the processing temperature of soda-lime glass is almost 1,000 degrees below that of pure quartz glass.

For glass production, the raw materials are mixed and heated in a refractory-lined tank. When the temperature is high enough, the mixture begins to melt. To drive out the gases generated by the melting process, the temperature is raised further (“refining”). This prevents the formation of glass defects due to air inclusions (bubbling). The melt is then allowed to cool down to its processing temperature (“stand-off”) – and the glassmakers or the machines for the production of flat glass go to work.

Flavio G.: „Why does glass break?“

Dr. Alexander Stoppa, Technischer Leiter:

„Glass is a very hard material. When subjected to pressure, it is more than twice as resistant as steel. The problem is tensile loads. At the bottom of micro-cracks, for example caused by mechanical damage to the surface, or structural defects in the glass, strong mechanical stresses arise. These then lead virtually instantaneously to breakage, which – as with other amorphous or brittle materials – hardly announces itself in advance. The technical strength of glass is therefore not an absolute value but is essentially determined by surface defects.

Many types of glass can be protected by thermal toughening. The processes involved make glass significantly less sensitive, which is why one speaks colloquially of “hardened glass”, similar to steel. If thermally toughened glass breaks, it will break into small pieces without sharp edges – in contrast to, for example, uncured wine glasses or bottles.”

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